There is broad interest in the application of Nuclear Medicine (NM) techniques with compounds of various specificity to functional imaging of breast lesions. The use of these compounds can be for confirmation of metastases based on the functional information, facilitating identification of cancerous lesions in women with large or especially dense breasts which particularly cause diagnostic problems in x-ray mammographic screening, and also as a way to monitor any treatment or therapy the patient receives. The efficacy of single photon emitting tracers versus positron emitting tracers remains debatable, yet the high sensitivities and specificities (˜90%) already achieved with breast imaging for ≧1 cm diameter lesions, as well as commercial availability of agents specifically targeted for breast tumor imaging, lend credence to the efficacy of the use of these various compounds. For example, in studies of women with suspicious mammograms, 2-dimensional planar imaging of ≧1 cm diameter breast tumors using single photon emitting 99mTc-labeled sestamibi or 99mTc-methylene-diphosphonate achieved sensitivities and specificities of ˜90%. While these results are encouraging in the specific population sample, dedicated 3-dimensional NM tomographic imaging with single photon emission computed tomography (SPECT) or positron emission tomography (PET), with superior lesion contrast and signal-to-noise ratio (SNR) characteristics could further improve diagnoses for this group, and potentially be applied more generally.
Conventional whole body SPECT imaging for breast disease is primarily limited by the resolution degradation resulting from the necessarily large radius of rotation (ROR) required to rotate the large and bulky camera system about the patient. Furthermore, additional problems include torso attenuation, primary and scatter contamination from cardiac or hepatic uptake of the tracer, as well as increased breast lesion scatter itself with supine dependant imaging of the breast. Tomographic, whole body SPECT imaging (3-dimensional, multiple projection angles) for breast imaging compared with planar scintimammography has yielded poorer imaging, and hence poorer diagnostic results than expected despite the improved lesion contrast and SNRs otherwise expected with SPECT. Thus, there is currently a strong notion that there is little added utility in conventional SPECT imaging for breast disease, despite the fact that there should, in principle, be much higher contrast of deeply seated lesions, especially with correctly applied, dedicated tomographic imaging techniques. Thus, the main reasons for these shortfalls are that (1) whole body SPECT requires large ROR, which substantially degrades spatial resolution for (small, <1 cm diameter) lesion visualization; (2) for posterior camera locations, the body attenuates the signal coming directly from the breast; and (3) there is substantial contamination of the lesion signals from cardiac and hepatic signals (primary and scattered radiation) where the 99mTc-labeled radiopharmaceutical compounds are also taken up, resulting in artifacts throughout the breast in reconstructed images.
Due to the increasing intensity in radiochemistry with PET radiopharmaceuticals and their growing availability from localized distribution centers throughout the United States, there is also great interest in the detection of coincident photons with whole body PET scanners. However, the clinical results with whole body PET are similar to those with whole body SPECT in that the effects of photon attenuation and scatter from the torso cause image artifacts and hence the potential for missed small breast lesions.
In contrast to current clinical whole body imaging protocols for evaluating breast disease, there are various dedicated NM breast imaging approaches currently under investigation with PET and SPECT. The simplest approach utilizes planar, single photon imaging (2-dimensional, single projection angle) with clinical gamma cameras (≧800 cm2 detector surface area) using various types of collimators. This basic approach, with very fine resolution parallel hole single photon collimators yielded the ˜90% sensitivity and specificity results for NM breast lesion detection and visualization described above.
Some dedicated breast SPECT approaches utilizing clinical gamma cameras with prone dependant breast have demonstrated that application specific tomographic imaging of the breast compared with planar imaging may provide improved images of breast lesions. See, e.g., Li et al., Limited angular view MLEM pinhole SPECT for breast tumor detection. J Nucl Med. 37(5):214P; Scarfone et al., Breast tumor imaging using incomplete orbit pinhole SPECT: a phantom study. Nuc Med Commun. 18:1077-1086; Wang et al., Prone breast tumor imaging using vertical axis-of-rotation (VAOR) SPECT systems: an initial study. IEEE Trans Nucl Sci. 44(3):1271-1276; and La Riviere et al., Ideal-observer analysis of lesion detectability in planar, conventional SPECT, and dedicated SPECT scintimammography using effective multi-dimensional smoothing. IEEE Trans Nuc Sci, 45(3):1273-1279, 1998. The various dedicated breast SPECT studies that employed clinical gamma cameras were, however, still limited by the large detector sizes that cannot achieve close proximity to the breast volume of interest. Since spatial resolution rapidly falls off with increasing distance in single photon imaging, these systems are limited in the object sizes that they can resolve. Even those systems that employ pinhole collimators, which generally have better sensitivity and resolution than parallel hole collimators at small separations or ROR, are limited in resolution since the breast volumes are not necessarily “small”, and there is severe axial blurring and other sampling artifacts which may limit the usefulness of the data to relatively small breast volumes.
Dedicated, small area gamma cameras (≦400 cm2 in area) have further demonstrated improved visualization of small tumor phantoms in compressed breast, planar geometries but are limited by low image contrast resulting from planar imaging, and, additionally, cannot provide 3-dimensional localization within the breast volume.
Some dedicated coincidence devices have been proposed for Positron Emission Mammography (PEM, which is a limited angle, non-fully tomographic cousin of PET) and successfully implemented on clinical x-ray mammographic devices so that there is inherent coregistration between x-ray mammograms and the functional PEM data. Furthermore, while full PET ring devices have been proposed, the dedicated devices have to date all been implemented in a static, approximately coplanar mode with opposed detector plates of various geometry. These approaches have limited quantitative and depth information in the volume of the (un)compressed breast geometries investigated and are akin to the single photon planar imaging approaches. It is unclear if these devices will prove clinically efficacious due to their inherent limitations.
In view of the shortfalls of the above noted imaging techniques, an object of the invention was to design a tomographic gantry for imaging metabolically active lesions in the pendant breast. This system overcomes physical constraints associated with imaging a pendulous breast in prone patients, while simultaneously satisfying sampling criteria for sufficient data collection in the pendulous breast reference frame. Thus, in one embodiment, the invention provides a compact and mobile gantry for 3-dimensional Application Specific Emission and/or Transmission Tomography (ASETT) imaging of the breast in single photon or coincidence emission modes, and single photon, coincident photon, or x-ray transmission modes.
More generally, the invention is embodied in an imaging system for generating images of a body part suspended within an imaging area of the system, comprising a support having a rotation axis extending through the imaging area and at least one imaging device having an imaging device axis which passes through a first imaging device field of view, the imaging device being mounted to the support so as to be selectively movable in three dimensions, including radial movement relative to the rotation axis, rotational movement about the rotation axis, vertical movement parallel to the rotation axis, and pivoting movement about a pivot axis perpendicular to the rotation axis, whereby the imaging device can be selectively moved along a path that defines a curved 3-dimensional surface. In an exemplary embodiment, the imaging device axis is laterally offset from the rotation axis and the support is mounted for rotational movement through at least about 180 degrees, whereby when the body part is greater than the imaging device field of view, an entire volume of the body part can be sufficiently sampled to accurately reconstruct the emission activity distribution.
The results of preliminary work with the system of the invention demonstrate the feasibility of a single compact emission imaging camera mounted on a versatile gantry to image the breast and associated axillary region. This work can be extended to include coincident detector systems placed on the gantry of the invention and used to acquire PET images of the breast. Furthermore, an x-ray transmission imaging system for dedicated breast computed tomography (CT) is also viable for use with this ASETT system, and its novel features are described.
Furthermore, fully tomographic transmission data (3-dimensional) which differs from partial view planar scans (2-dimensional) can also be used in both SPECT and PET for attenuation correction of the emission data. This highly accurate structural transmission map ultimately leads to more quantitatively accurate functional data from which parameters like metabolic rates of reaction can be determined to monitor therapeutic progress and determine tissue necrosis versus tumor recurrence in a patient. Simply having a structural framework (the structural x-ray CT image) with which to identify the location of the focal radioactive uptake with NM imaging (often a diffuse or ambiguously localized region of greater signal) may be enough to aid in breast lesion image assessment alone.
Due to some physical constraints associated with imaging a single pendant breast with maximal separation from the nearby body containing background, SPECT techniques which employ cameras whose line-of-sight of the activity distributions are determined by collimators of various solid geometries (e.g. parallel beam, fan beam, cone beam, pinhole, slanted, angled, etc.) may have some physical advantages compared with PET techniques. Moreover, placing dedicated SPECT cameras in close proximity to the breast (or other object of interest, e.g. the prostate) to fully sample the object volume is critical to obtaining complete data for quantitatively imaging small lesions and/or lesions with low radiopharmaceutical uptake, which is ultimately a determining factor in fully exploiting the power of functional imaging and volumetric localization in the breast or other organs.
There are various anticipated advancements gained with a high performance, dedicated tomographic system embodying the invention including improved SNR and contrast characteristics due to (1) the improved intrinsic spatial and energy resolution potentially afforded by dedicated, compact, high performance imaging systems which can therefore minimize scatter contamination, (2) the closer achievable proximity to the object of interest with more compact imaging systems which improves collimator-limited spatial resolution for SPECT, and (3) due to (2) the camera will preferentially view the breast and minimally view signals from other regions of the body. These advancements should result in an ability to image and 3-dimensionally localize smaller (<1 cm diameter), non-palpable and potentially pre-metastatic tumors in a larger population with smaller variance and bias. The use of multiple ASETT scans over time with NM techniques can guide treatments, monitor therapy, and help evaluate outcomes. The use of combined structural and functional imaging may help even further in patient management and care.
Both the structural and functional volumetric information could potentially be used to guide needle biopsies more accurately than with current planar approaches which have limited depth information; more accurate needle guidance could improve the needle localization, hence lower false positives, and overall improve diagnosis and guide decisions about treatment protocols for patients.